This invention relates to a method for remediating soil contaminated with radioactive species. This invention further relates to a method for remediating uranium and radium contaminated soil by selectively removing the radioactive contamination without removing desirable organic material which enriches the soil for subsequent plant growth. The method of the invention results in a leachate solution which is amenable to further treatment and reuse.
The contamination of soils, ores or other materials with radioactive species, such as uranium, radium, and thorium, is a common environmental problem. In a great number of cases, the radioactive contamination is distributed throughout various fractions routinely found in soil, including gravel, sands, clays and silt, vegetation, organic matter, and groundwater. Radioactive contaminants may be found in mining sites, radioactive processing facilities, and even residential areas. In all cases it is necessary to remove or clean up the soil or other material.
Two approaches typically are used to treat contaminated soil. The first approach involves methods for preventing or restricting the dispersion of the contamination to the immediate surroundings. However, the application of such methods is problematic, since any breakdown is likely to result in widespread leakage of contaminants to adjacent land and water areas by groundwater, and can have a deleterious effect on flora, fauna and humans.
The second approach involves methods for removing or destroying contamination from soil, also referred to as "cleaning" or "remediating" the soil. One suitable remediation technique involves excavating the contaminated soil and reburying it in another area. But, the practice of digging the soil up in one area only to bury it in another, is expensive and becoming less environmentally acceptable.
Other techniques have been developed for remediation on-site or elsewhere. For example, U.S. Pat. No. 4,783,253, issued to Ayres et al., describes a method for treating water insoluble contamination which uses a concurrent flow of water to float away lighter uncontaminated particles from heavier contaminated particles. The slurry of particles is dewatered using a spiral classifier, centrifuge, filter or the like.
Heavy metal contaminants, including radioactive species, can also be removed from soil by size separation techniques, such as tiltable tables, or concurrent flow size separation in a mineral jig. Size separation and leaching techniques are described in U.S. Pat. No. 5,045,240, issued to Skriba et al., U.S. Pat. No. 5,128,068, issued to Lahoda et al., and U.S. patent application Ser. No. 07/722,458, filed Jun. 27, 1991, in the name of Grant, et al.
Leaching methods and soil washing methods are also used to remove radioactive species from soils. The use of aqueous extracting solutions to treat contaminated soils is described in "Extractive Methods For Soil Decontamination; A General Survey and Review of Operational Treatment Installations," J. W. Assink, in Contaminated Soil, Edited by J. W. Assink, and W. J. Van Den Brink, pp. 655-667, 1986, Martinus Nijhoff Publishers. Such solutions usually contain only water. Alternatively, they can contain bases like sodium carbonate or sodium hydroxide, or strong mineral acids like hydrochloric acid and nitric acid, to aid in the extraction process. After soil treatment, the contaminated extracting fluids can be cleaned with any known physical, chemical or biological purification methods. For instance, coagulation, flocculation followed by sedimentation, or flotation may be used. Other suitable techniques include ion exchange, aerobic and anaerobic biological purification, electrolysis, and membrane filtration.
There are several limitations associated with the above mentioned techniques. Each of the methods described above strips the soil of valuable organic material like humus which is crucial for subsequent plant growth. Moreover, depending upon the treatment process used, the presence of soluble organics in the extracting solution may interfere with the subsequent removal of the radioactive contaminant from solution. In particular, the presence of organic matter reduces the capacity of ion exchange resins which typically are used to remove radioactive species from the extracting solution. In addition, such methods do not address treatment of fine vegetation, such as root hairs, which also are likely to be contaminated.
Another disadvantage of the above mentioned remediation methods is that they are not suitable for all types of soil. Loamy soil, clay, and peat sites are generally difficult to clean by extraction. Humus-like components, silt, and clay particles readily form relatively stable suspensions with the extraction liquid. This is especially true for aqueous extracting solutions having a high pH. If the contaminants are present in the extracting solution as separate small particles, it is often impossible to separate relatively clean soil particles from the contaminated particles and effluent.
What is needed is a simplified method of treating large volumes of materials of different types, and containing soluble portions of hazardous or radioactive waste, so as to segregate the clean from the contaminated material and to concentrate the contaminated material. In particular, the method should successfully remove the radioactive contaminants from different types of soil without removing desirable materials.
There is a further need for a system that can effectively recover the contaminants once they have been removed from the soil, requiring a minimal amount of equipment, chemicals, and which further allows for the processing of recovered contaminants, such as metals, or other salable minerals.